In interpretation of signals received from a wide variety of receptors, (e.g., television cameras, x-ray scanning devices, radar), it is desirable to process the signals in such a manner as to produce a processed and filtered image capable of representation in a more specifically usable form, for example, as a visible display on a conventional video display terminal. It is also desirable to precisely control the properties of individual aspects of the displayed image. These systems, referred to as hybrid-optical systems have the potential to gain an important role in signal processing due to their compactness, efficiency and high information rate of processing. Recently, high power, single-mode diode laser systems have become available enabling the useful operation of a certain class of hybrid-optical interferometric processors. In this type of processor, the diffracted and undiffracted orders of an acousto-optic cell are interfered. Due to the relative Doppler frequency shift between the orders, the interference fringes are modulated at the Doppler frequency. This typically would cause the destruction of fringe visibility when observed with time integrating detectors such as television cameras.
Restoration of the fringe visibility can be achieved by shifting in frequency one of the two orders of the light from the acousto-optic cell. For example, a second acousto-optic cell may be utilized and modulated at the same frequency as the original. However, this solution is not particularly desirable, due to the additional signal processing circuitry, acousto-optic cell, and associated optics which make this methodology unduly complex.
Any laser may be utilized with external modulation sources. However, because of their bulk, high cost, and necessity for external modulation continuous wave gas lasers are not well suited to compact installations. Similar limitations apply to other types of lasers, for example dye lasers and optically pumped solid state crystalline or glass lasers. Such lasers may also be utilized with external modulation sources. In contrast, solid state diode lasers offer the benefit of direct modulation, compact size and ruggedness.
The present invention overcomes these shortcomings and provides a lower cost, rugged, compact and simplified method of improving fringe visibility .